小肠（一）： 结构

did you know that the stomach flu can make you temporarily lactose-intolerant it's true so in this video we'll talk about how that happens and in detail how the small intestine works so as a review once you put food into your oral cavity or your mouth its chewed up and then sent down the esophagus where it ends up in the stomach where it's churned and then introduced to acid where it gets broken down into chyme and then delivered into the first part of your small intestine now the small intestine has three different parts to it so let's take a better look so the first part of the small intestine is called the duodenum the duodenum this receives the chyme that just got processed in the stomach and it's the part of the entire GI tract where the most digestion occurs the most breakdown of food products will happen in the duodenum alright so the next part of the small intestine is called the jejunum I'll just write that right here the jejunum and this is the part of the entire GI tract where the most absorption occurs anywhere so the most absorption of nutrients is going to happen in your jejunum the jejunum then finally after your food passes to the jejunum it gets to the last part of the small intestine and that's called the ileum the ileum IL e um and the ileum now this doesn't have a superlative like the most digestion or the most absorption but there are some pretty important things that are absorbed here things like vitamin b12 vitamin A D ek so there are some important things that are absorbed here so I'm just gonna write important absorption there are some important things that are absorbed in your ileum now the busiest part of your small intestine is the duodenum because there are a bunch of things that are involved in this digestion process so there are four key things to keep in mind first of all your stomach is going to be delivering a bunch of time or processed food into the duodenum so you're going to be working with all I'm here in addition you're gonna have some stomach acid that processed food into chyme that's going to be present in the duodenum in addition to the stomach the liver and the gallbladder are also going to be important to deliver bile to your duodenum so they give bile and as I'll talk about in a subsequent video bile is composed of two different things bile salts and bile pigments and beyond the liver and the gallbladder the pancreas also delivers a couple of very important enzymes for digestion here so I'm just gonna write enzymes for now and in a minute I'm gonna go through and talk about which enzymes are delivered by the pancreas and then finally the duodenum itself has what are called brush border enzymes brush border enzymes that are very important for activation of certain enzymes and also for digestion of several nutrients that we're going to discuss so let's talk a little more about this brush border now if I were to make a little drawing of the duodenum right here remember that first part of our small intestine I would draw just this little tube connected right there and then blow up the wall if I want to take a better look at what's going on right there we would find then that there's a whole bunch of things than just meets the eye first of all the wall isn't just a straight line there's actually a bunch of in foldings that are present on the wall to help increase surface area think about it if we're trying to digest as much as we can here we need to make sure that there are a lot of projections or a lot of space where we can make contact with the food that's passing by so if this is the inside of our duodenum and this is the outside just like how we drew up here that's in and that's out you can notice that this wall here has a whole bunch of projections on it these projections are called villi VI ll eye villi and a single one of them is just called a villus just a single villus and these are just a couple of folds or these outpouching x' that help increase the surface area of our duodenum now that's not where the story ends if we take a closer look at one of these villi then we'd find that there are even more projections sitting on that even smaller microscopic projections so for this single villus that had a horizontal line right here for its shape if I were to draw it out here you would notice that it's not a straight line but instead these also have a bunch of projections that are present on them and if you were to guess why all these projections aren't there I'm sure you'd say to further increase our surface area that's sort of the name of the game when we're in the small intestine to increase our surface area and so these little guys cutely enough are called micro villi micro villi and a single one is just called a micro villus so we've got these villi right here or the single villus that you can see if you just blow up the wall of the duodenum and then if you blow up a single villus you'll find that they have a whole bunch of micro villi that are found on them too to increase our surface area because that allows for better digestion and when we say brush border enzymes these are a whole bunch of enzymes that are present on this brush border because think about it these villi and these micro villi they're no different from bristles on a comb they act to increase surface area or places where you're going to have interaction with food that you want to digest and so there are enzymes that are present on this brush border so just to make the point all of these micro villi and villi together that's what makes up the brush border of our duodenum the brush border which is the increased surface area of the wall helps to digest food with our brush border enzymes that are present and as we'll see later in the jejunum it helps for absorption so now let's talk about the digestion process in detail